AMTrans - A most simplistic AM radio transmitter
author: BattleToad
2 layer board of 2.22 x 1.26 inches (56.4 x 32.0 mm)
Uploaded:
March 29, 2020
Shared:
May 29, 2020
Total Price:
$14.00
BOM
Contact me if there are questions.
Recently, when I started playing around with detector radios from the Twenties, I painfully realized one thing: There are hardly any stations to listen to - at least that’s the case here in Germany where I live. Instead of waiting till after midnight for catching an oddball station under lucky conditions, I rather wanted to broadcast some audio content from my mp3 player or cellphone on a short ranged AM radio transmitter.
Browsing the web I found this website which describes a simple transmitter kit that operates on the medium wave band. The underlying principles of this circuit are not explained here. The AMTrans project is a re-interpretation of this idea with 0603 and SOT-23 components.
After some research I found out that this circuit essentially consists of a Colpitts oscillator which generates the carrier wave. The necessary amplification is provided by a PNP transistor in common base configuration, which means that the feedback network doesn’t have to add an extra phase. The antenna signal is tapped off the feedback line starting from between the pair of capacitors which is characteristic of Colpitts' design. The audio input is coupled into the emitter and regulates the PNP transistor’s gain so that the carrier wave gets modulated by the input signal.
Although the range of this transmitter only covers a few meters, please keep in mind that even the operation of a weak radio transmitter like this may be illegal in your country. Hence, let me cite Paul Carlson: “If you are following along you are doing so at your own risk…” ;-).
Schematics:
3D Model:
BOM
Contact me if there are questions.
Recently, when I started playing around with detector radios from the Twenties, I painfully realized one thing: There are hardly any stations to listen to - at least that’s the case here in Germany where I live. Instead of waiting till after midnight for catching an oddball station under lucky conditions, I rather wanted to broadcast some audio content from my mp3 player or cellphone on a short ranged AM radio transmitter.
Browsing the web I found this website which describes a simple transmitter kit that operates on the medium wave band. The underlying principles of this circuit are not explained here. The AMTrans project is a re-interpretation of this idea with 0603 and SOT-23 components.
After some research I found out that this circuit essentially consists of a Colpitts oscillator which generates the carrier wave. The necessary amplification is provided by a PNP transistor in common base configuration, which means that the feedback network doesn’t have to add an extra phase. The antenna signal is tapped off the feedback line starting from between the pair of capacitors which is characteristic of Colpitts' design. The audio input is coupled into the emitter and regulates the PNP transistor’s gain so that the carrier wave gets modulated by the input signal.
Although the range of this transmitter only covers a few meters, please keep in mind that even the operation of a weak radio transmitter like this may be illegal in your country. Hence, let me cite Paul Carlson: “If you are following along you are doing so at your own risk…” ;-).
Schematics:
3D Model:
